The ruling on voluntary charity given by one who owes zakah

Question:
He says: If I always give charity and I have zakah that I have not paid, is it considered charity or zakah?

Answer:
No, it is not considered zakah, because voluntary charity does not suffice for the obligatory zakah. One must calculate the zakah that is due from him and pay it immediately. This voluntary charity has a different status; he will be rewarded for it, if Allah wills. Because one who intends a voluntary act does not have it count for the obligatory act. One who intends a voluntary act—this is a general rule in all acts of worship. One who intends a voluntary act does not have it count for the obligatory act, except in the 'Asr prayer, where one who intends the obligatory prayer has it count for the voluntary prayer if he intended it. Yes.